Background Little is known about the impact of diastolic interventricular septal flattening on the clinical outcome in patients with severe tricuspid regurgitation. This study sought to evaluate the association of diastolic interventricular septal flattening with clinical outcome in patients with severe tricuspid regurgitation. Methods and Results We retrospectively studied 407 patients who underwent 2‐dimensional transthoracic echocardiography and were diagnosed with severe tricuspid regurgitation between January 2014 and December 2015. Cardiovascular events were defined as cardiovascular death or admission for heart failure. The magnitude of interventricular septal flattening was calculated by the eccentricity index (EI) of the left ventricle, and hemodynamic parameters were obtained from transthoracic echocardiography. During follow‐up (median, 200 days; interquartile range, 35–1059), 117 of the patients experienced cardiovascular events. By multivariate analysis including potential covariates, EI at end‐diastole and left ventricular ejection fraction were independent predictors of cardiovascular events (hazard ratio, 5.33 [1.63–17.41]; hazard ratio, 0.98 [0.97–0.99], respectively). An EI of 1.2 at end‐diastole was the optimal cutoff value for identifying poor hemodynamic status defined as cardiac index ≤2.2 L/min per m ² and right atrial pressure 15 mm Hg, both on transthoracic echocardiography. Patients with D‐shaped left ventricle defined as EI ≥1.2 at end‐diastole showed worse outcomes than those without (adjusted hazard ratio, 1.80 [1.18–2.74]). Conclusions Increasing EI at end‐diastole was strongly associated with worse outcomes in patients with severe tricuspid regurgitation. Furthermore, the presence of D‐shaped left ventricle defined as EI ≥1.2 at end‐diastole provides prognostic value for cardiovascular events.